Oxygen is produced in air separation plants that employ a cryogenic rectification process to separate the air into its component parts. In such a plant, the air is compressed, purified and cooled within a main heat exchanger to a temperature suitable for its rectification within distillation column. Typically, such plants utilize an air separation unit having higher and low pressure distillation columns that are so designated in that the high pressure column operates at a higher pressure than the low pressure column. The compressed, purified and cooled air is introduced into the high pressure column to produce a crude liquid oxygen column bottoms, also known as kettle liquid. The crude liquid oxygen column bottoms is further refined in the low pressure column to create an oxygen-rich liquid column bottoms. The oxygen product can be taken as a liquid from such liquid column bottoms.
In a common type of air separation plant, the higher and low pressure columns are thermally linked by a condenser reboiler situated in the base of the low pressure column. A stream of nitrogen-rich vapor is withdrawn from the top of the high pressure column and is condensed within the condenser reboiler against vaporizing part of the oxygen-rich liquid collected in the bottom of the low pressure column. The condensed nitrogen-rich vapor is used to reflux both the higher and low pressure columns and may be taken in part as a product. Again, typically, nitrogen-rich vapor, produced as column overhead in the low pressure column, a stream of the oxygen-rich liquid column bottoms of the low pressure column and an impure nitrogen stream withdrawn from below the top region of the low pressure column are all introduced into the main heat exchanger to cool the air and to vaporize the oxygen-rich liquid and produce the oxygen product.
Where the oxygen product is desired at pressure, the oxygen-rich liquid produced in the low pressure column is pumped and then vaporized in main heat exchanger through indirect heat exchange with part of the air that has been compressed to a sufficiently high pressure for such purposes. The vaporization of the pumped liquid results in liquefaction of the compressed air to produce a subcooled liquid air stream. The liquid air stream after having been expanded to a suitable pressure of the low pressure column is introduced into the low pressure column. Part of such stream can also be suitably expanded and then introduced into the high pressure column. The introduction of the liquid air into the distillation columns, particularly the low pressure column, has the effect of increasing the recovery of the oxygen and also the argon where an argon column is connected to the low pressure column to produce an argon product.
The problem in conducting the vaporization of the pumped oxygen-rich liquid and the liquefaction of air entirely within the main heat exchanger is that a considerable length of the main heat exchanger is taken up in the transfer of latent heat for the vaporization of the oxygen-rich liquid and the liquefaction of the air. This leads to higher fabrication costs of the main heat exchanger. At the same time, since all passages within the main heat exchanger can become longer for such purposes, there are increased pressure losses within the main heat exchanger and therefore, increased power costs in the compression of the air. In order to overcome these problems, it is known to employ an auxiliary heat exchanger in which all latent heat transfer takes place between the pumped oxygen-rich liquid and the compressed air. Additionally, sensible liquid heat transfer also takes place within the auxiliary heat exchanger after liquefaction of the air and the oxygen-rich liquid prior to its vaporization. After vaporization of the oxygen-rich liquid, the resulting vapor is further warmed to ambient through indirect heat exchange of sensible heat with the compressed air entering the warm end of the main heat exchanger. While this arrangement incorporating the auxiliary heat exchanger results in a shorter heat exchanger, the degree to which the liquid air is subcooled is very limited given the amount of air that must be consumed in vaporizing the liquid oxygen and the amount of sensible heat that can be transferred from the oxygen-rich liquid to the liquid air within the auxiliary heat exchanger. Unfortunately, the degree to which the liquid air is subcooled will have an effect on oxygen and potentially argon recovery, if present, given that the colder the liquid air upon entry to the low pressure column, the greater degree to which oxygen and argon is driven down the column.
As will be discussed, the present invention provides a method and system for vaporizing a pumped liquid oxygen stream that utilizes an auxiliary heat exchanger in a manner in which the liquid air is introduced into the low pressure column and also the high pressure column at a lower temperature than that possible with the use of an auxiliary heat exchanger arrangement of the prior art.